An evaluation of absorption spectroscopy to monitor YBa₂Cu₃O₇₋[x] precursors for metal organics chemical vapor deposition processing

Absorption spectroscopy was evaluated as a technique to monitor the metal organic chemical vapor deposition (MOCVD) process for forming YBa₂Cu₃O_7-x superconducting coated conductors.Specifically, this study analyzed the feasibility of using absorption spectroscopy to monitor the MOCVD supply vapOr concentrations of the organic ligand 2,2,6,6-tetramethyl-3,5-heptanedionate (TMHD) metal chelates of barium, copper, and yttrium.Ba(TMHD)₂, Cu(TMHD)₂, and Y(TMHD)₃ compounds have successfully been vaporized in the MOCVD processing technique to form high temperature superconducting "coated conductors," a promising technology for wire fabrication.The absorption study of the barium, copper, and yttrium (TMHD) precursors was conducted in the ultraviolet wavelength region from 200nm to 400nm. To simulate theMOCVD precursor flows the Ba(TMHD)₂, Cu(TMHD)₂, and Y(TMHD)₃ complexes were vaporized at vacuum pressures of (0.03-10)Torr. Spectral absorption scans of each precursor were conducted to examine potential measurement wavelengths for determining vapor concentrations of each precursor via Beer's law.The experimental results show that under vacuum conditions the barium, copper,and yttrium (TMHD) precursors begin to vaporize between 90°C and 135°C, which are considerably lower vaporization temperatures than atmospheric thermal gravimetric analysis indicate. Additionally, complete vaporization of the copper and yttrium (TMHD)precursors occurred during rapid heating at temperatures between 145°C and 195°C and after heating at constant temperatures between 90°C and 125°C for approximately one hour,whereas the Ba(TMHD)₂ precursor did not completely vaporize. At constant temperatures,near constant vaporization levels for each precursor were observed for extended periods of time. Detailed spectroscopic scans at stable vaporization conditions were conducted.